The attacks were aimed at a Georgian blogger with accounts at the various affected sites, according to a report on Cnet, the technology news site.

Vulnerable sites 

Barry Fox, an internet security and technology analyst, told Al Jazeera: "It does look very much as if this can't be a coincidence that you've got the Russia-South Ossetia-Georgia flare-up again, the year anniversary, and all this happens ... on the Twitter and Facebook sites where there was a very active anti-Russian blogger.

"History would tell us that it's probably the same attacker or group of attackers that is launching both attacks"

Kevin Prince, chief technology officer of  Perimeter eSecurity

"It does look as if trying to silence one person has effectively silenced the whole sites."

"Hackers take over computers which are used in homes around the world and turn those computers into zombies.

"Then, at a pre-appointed hour, one hacker somewhere in the world effectively presses a button  ...  and all those zombie computers start trying either to access the target website, or sending masses of emails, or spam."

Biz Stone, Twitter's co-founder, said in a blog the company preferred not to speculate about the motivation behind the malicious attack.

"Twitter has been working closely with other companies and services affected by what appears to be a single, massively co-ordinated attack," said Stone.

The incidents have underscored the vulnerability of internet social networking sites that are now being used to counter censorship and authoritarianism.

Expensive defence

They followed a similar online assault a month ago which targeted the White House website.

Security experts said a single group could have been behind the problems on Twitter, Facebook and the other sites as hackers evolve their ability to attack multiple sites at once.

"History would tell us that it's probably the same attacker or group of attackers that is launching both attacks," Kevin Prince, the chief technology officer of security services provider Perimeter eSecurity, told the Reuters news agency.

While there are ways for websites to protect themselves from denial of service attacks, Prince said the defences were expensive, whereas mounting an attack was a relatively simple feat for hackers.

Twitter's newfound fame makes it an easy target for hackers, said Steve Gibson, the president of internet security research firm Gibson Research Corp.

The number of worldwide unique visitors to the Twitter website reached 44.5 million in June, up 15-fold year-over-year, according to comScore data.

Twitter, which allows people to broadcast short, 140-character text messages over the internet, became a key form of communication in Iran amid the protests and clampdown that followed the country's disputed June elections.